The Anattalakkhaṇa Sutta (SN 22.59) was the Buddha’s second discourse, given to the same five monks a few days after the first sermon. It explains the characteristic of anattā — not-self.
The five aggregates
The Buddha analyses a person into five aggregates (khandhas): form, feeling, perception, mental formations and consciousness. He points out that if form were truly self, it would not lead to affliction, and we could say “let my body be like this, let it not be like that”. Since we cannot, form is not self — and the same is true of the other four.
Impermanent, unsatisfactory, not-self
He then asks: is form permanent or impermanent? Impermanent. Is what is impermanent satisfying or unsatisfying? Unsatisfying. Is it fitting to regard what is impermanent, unsatisfying and subject to change as “this is mine, this I am, this is my self”? No.
Seeing this clearly, a wise disciple becomes disenchanted, dispassion arises, and the mind is liberated. The sutta records that all five monks became fully awakened (arahants) while listening.
Reflection: notice a feeling as it arises and passes. Did you choose it? Could you keep it?